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Article

Green Regeneration and Resource Recovery of Nickel-Plating Waste Solution: A Synergistic Study of Electrodialysis and Advanced Oxidation

by
Xiaolong Xiong
1,†,
Kangping Cui
1,*,†,
Haiyang Li
1 and
Wenming Wu
2
1
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230000, China
2
Zhongxin Link Environmental Technology (Anhui) Co., Ltd., Lu’an 237000, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(7), 1071; https://doi.org/10.3390/w17071071
Submission received: 13 March 2025 / Revised: 28 March 2025 / Accepted: 2 April 2025 / Published: 3 April 2025
(This article belongs to the Topic Advanced Processes and Technologies for Wastewater: Collection, Treatment, and Resource)
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Abstract

:
Electroless nickel plating is a chemical deposition process in which nickel ions within a plating solution are reduced by a chemical reducing agent and subsequently deposited onto the surface of a solid substrate. Chemical nickel-plating wastewater contains substantial amounts of phosphorus as well as abundant nickel resources. In this study, electrodialysis coupled with advanced oxidation techniques was utilized for the efficient recovery of nickel and phosphorus from spent nickel-plating solutions. The end-of-life tank solution from chemical nickel plating was treated via electrodialysis to remove harmful phosphite and sulfate ions, enabling the purified solution to be reused in plating production by supplementing it with appropriate amounts of sodium hypophosphite and nickel sulfate. Subsequently, the concentrate generated from electrodialysis was treated using peroxydisulfate (PDS)-based advanced oxidation technology to break nickel complexation and simultaneously promote the oxidation of hypophosphite and phosphite ions. Finally, Ca(OH)2 was employed as a precipitating agent to effectively recover phosphorus from the treated concentrate. From an economic perspective, optimal process conditions were determined as follows: a current density of 20 mA/cm2, concentrate-to-dilute water volume ratio of 1:1, current speed of 1.0 m3/h, and a sodium sulfate concentration in concentrate of 20 g/L. Under these conditions, the migration rates of H2PO2− and HPO32− ions reached 67.3% and 62.53%, respectively, whereas Ni2+ exhibited significantly lower mobility at only 6.77%. The purified wastewater recovered approximately 60% of its initial plating activity. Regarding the concentrate—which is a by-product of electrodialysis—the hypophosphite ions were nearly completely oxidized using a PDS dosage of 0.3 mol/L. Furthermore, when the Ca/P molar ratio was adjusted to 2.0, total phosphorus (TP) and nickel (Ni) removal efficiencies exceeded 98% and 93%, respectively.

Graphical Abstract

1. Introduction

Chemical nickel plating has been widely applied in the surface treatment industry due to its relatively simple equipment requirements, the elimination of an external power supply, auxiliary electrodes, and transmission systems, and advantages such as uniform plating thickness, excellent adhesion, superior mechanical properties, and high coating hardness [1,2]. Hypophosphite-based electroless nickel-plating solutions represent the predominant industrial choice, typically employing nickel sulfate as the main metal salt, sodium hypophosphite as a reducing agent, citric acid or its salts as complexing agents, sodium acetate as a buffering agent, and minor additives to facilitate nickel deposition through redox reactions [3,4]. However, this process has significant drawbacks. With ongoing plating reactions, by-products such as phosphite continuously accumulate, causing the performance deterioration of the plating bath. When the phosphite concentration reaches a certain threshold, the plating solution loses effectiveness and becomes a waste solution [5]. Typically, such waste solutions contain 10–200 g/L phosphorus, 2–7 g/L nickel, more than 20 g/L sodium sulfate, trace organic substances, and impure metal ions [6]. It has been reported that the service life of existing chemical nickel-plating baths typically does not exceed 10 Mean Times of Solution (MTOS) [7]. Additionally, the disposal cost of chemical nickel-plating wastewater is notably high; previous studies indicated that the treatment cost for 1 m3 of the waste solution ranges from approximately 2700 to 4000 RMB, significantly burdening enterprises in the competitive surface-treatment industry [8]. Due to its toxic constituents, chemical nickel-plating wastewater poses severe environmental risks. Nickel and nickel compounds significantly threaten aquatic ecosystems and human health, while phosphorus contributes to eutrophication, potentially triggering algae blooms, cyanobacteria proliferation, and red tides [9,10,11]. Moreover, nickel, a valuable yet expensive metal, and phosphorus, a non-renewable resource, have economic importance that mandates their recovery. Traditional nickel recovery methods, such as sludge formation, fail to achieve precise resource recovery and maximal utilization [12,13,14].
Researchers have long investigated various treatment technologies for chemical nickel-plating wastewater, including chemical precipitation [15], advanced oxidation processes [16,17], electrochemical methods [18], ion exchange [19], adsorption [20], membrane separation [21], and biological treatments [22]. The key achievements and shortcomings of existing processing technologies are summarized in Table 1. Previous studies also indicate that conventional precipitation is insufficient for complexed nickel removal, and hypophosphite treatment is problematic by traditional means. Only limited studies have explored the decomposition of complexed nickel species by hydrogen peroxide (H2O2). For example, Liang et al. developed CuO-CeO2-CoOx composite nanocatalysts for a heterogeneous Fenton-like process, achieving over 99.9% removal of nickel(II)–citrate complexes, but this utilized a relatively low initial nickel concentration (1.0 mM) without addressing the oxidation of hypophosphite and phosphite [23]. In recent years, numerous researchers have explored regeneration and reuse technologies for electroless nickel-plating wastewater. Among these, electrodialysis (ED) has gained widespread application due to its excellent ion selectivity. ED is a membrane separation technology that utilizes an electric field and ion-exchange membranes to selectively transport and separate ions from a solution [24]. It plays a crucial role in desalination, ion removal, and concentration-based separation processes. Song et al. employed ED under conditions of 65 mA/cm2 current density, 37 °C temperature, 5 cm membrane spacing, and 0.02 L/s current speed for 48 h, achieving approximately 60.86% removal of phosphite, but incurring losses of 63.13% hypophosphite and 1.2% nickel ions [25]. Moreover, the concentrated wastewater generated by ED treatment remained inadequately addressed, limiting its practical application.
Currently, advanced oxidation methods employing strong oxidants have emerged as effective solutions for high-concentration hypophosphite and phosphite wastewater [26,27,28]. Several studies report the successful oxidation of hypophosphite and phosphite using peroxymonosulfate (PMS)-based processes. For instance, Dong et al. achieved hypophosphite (H2PO2−) and phosphite (HPO32−) conversion rates of 100% and 99.69%, respectively, via PMS activation combined with alkaline and thermal conditions [29]. Similarly to PMS and H2O2, PDS has recently attracted significant attention as a powerful oxidant [30]. Commonly employed activation methods for PDS include photocatalytic activation [31,32], thermal activation [33,34], transition metal catalysis [35,36,37], carbon–material catalysis [38,39,40], electrical activation [41], and alkaline activation [42,43]. However, each activation technique still faces challenges that necessitate further exploration and optimization.
In light of these challenges, the optimal treatment strategy involves selectively removing harmful sulfate and phosphite species while preserving other valuable components in the waste solution. The purified solution can then be reused after appropriate supplementation, significantly extending its operational lifespan, reducing environmental impacts, and lowering wastewater treatment costs. Therefore, this study proposes an innovative treatment strategy that couples electrodialysis with PDS-based advanced oxidation, maximizing nickel and phosphorus recovery from chemical nickel-plating wastewater. Specifically, the effects of key ED operating parameters on hypophosphite and phosphite ion transport were investigated to determine optimal conditions. Subsequently, the influences of PDS dosage and reaction conditions on hypophosphite oxidation, as well as the impact of Ca/P molar ratios on final effluent quality, were explored. Consistent with cleaner production principles, this study achieved the purification, reuse, and near-zero discharge of nickel-plating wastewater, providing practical and cost-effective solutions for enhanced resource recycling, reduced pollutant emissions, and lower wastewater treatment costs within the surface treatment industry.

2. Materials and Methods

2.1. Materials and Composition of Wastewater

The high-concentration hypophosphite and phosphite wastewater used in this study was obtained from an industrial chemical nickel-plating process. Wastewater composition analysis showed significant concentrations of phosphorus and nickel, accompanied by other constituents such as sulfate ions, organic impurities, and trace metal ions. The ED system and associated membranes were procured from Shandong Tianwei Membrane Technology Co., Ltd. (Weifang, China) Analytical chemicals, including sodium thiosulfate standard solution, iodine standard solution, starch indicator, and sodium bicarbonate, were purchased from Shanghai National Pharmaceutical Group Chemical Co., Ltd. (Shanghai, China) Other chemicals employed in this study were of analytical grade. The specific composition of the chemical nickel-plating wastewater is detailed in Table 2. Deionized water was used throughout the experimental procedures.

2.2. Experimental Steps

2.2.1. Regeneration of Wastewater by ED

A volume of 1 L nickel-plating wastewater was introduced into the dilute chamber of the ED apparatus, while the 1 L sodium sulfate solution at a concentration of 20 g/L was prepared and added into both the concentrate and electrode chambers. After the power supply was activated, the conductivity of the solution in the concentration chamber was continuously monitored. The electrodialysis process was terminated once the conductivity reached approximately 78 ms/cm, indicating the completion of the ion migration. The overall reaction duration was approximately 120 min. Each set of experiments was repeated three times to minimize experimental errors.

2.2.2. Reuse of Regenerated Wastewater

The regenerated electroless nickel-plating wastewater (700 mL) was adjusted to contain a nickel concentration of approximately 6.0 g/L and a hypophosphite concentration of 12 g/L by supplementation with nickel sulfate and sodium hypophosphite solutions. The solution was then diluted to 1 L with purified water, and ammonia was added dropwise to adjust the solution’s pH to approximately 4.6. The plating bath was maintained at a nickel concentration range of 5–6 g/L and a hypophosphite concentration between 18.75 and 22.5 g/L. After standard pretreatment procedures—including degreasing, rinsing, acid activation, and washing—the nickel-plating solution was heated to between 85 °C and 91 °C, and electroless nickel plating was performed on iron sheet samples. The plating experiments were carried out for four cycles, with each cycle consisting of three consecutive plating runs, each lasting 1 h. The performance of the regenerated plating bath was evaluated by comparing the plated samples against those obtained using a freshly prepared standard plating solution.

2.2.3. PDS Oxidation Experiment

The concentrated wastewater obtained from the ED process (100 mL) was subjected to advanced oxidation treatment using PDS. Different dosages of PDS were added to investigate the optimal oxidant concentration. The solution was continuously stirred during oxidation, and samples were collected every 10 min to evaluate the oxidation efficiency. Additionally, the effects of the reaction temperature and initial solution pH on the oxidation performance were systematically studied. The oxidation efficiencies of different oxidants were also compared under identical experimental conditions. Each set of experiments was repeated three times to minimize experimental errors.

2.2.4. Calcium Salt Dosing Experiment

Calcium hydroxide (Ca(OH)2) was employed as the precipitant to recover phosphorus from the oxidized wastewater. Experiments were conducted at room temperature (approximately 25 °C) to determine the optimal calcium-to-phosphorus (Ca/P) molar ratio. Various Ca/P ratios (1.0, 1.5, 2.0, 2.5, and 3.0) were tested to examine their effects on the removal efficiencies of phosphorus and nickel ions. Additionally, the influence of reaction temperature on phosphorus recovery was investigated, and comparative studies were carried out using different calcium salts to assess their effectiveness in removing phosphorus and other residual ions from wastewater. Each set of experiments was repeated three times to minimize experimental errors.

2.3. Analytical Methods

The concentrations of hypophosphite and phosphite ions were determined by standard titration methods [44]. Orthophosphate (PO43−) and TP concentrations were measured separately using ammonium molybdate spectrophotometry at a wavelength of 700 nm with a UV-Vis spectrophotometer (Model 752N, Shanghai Yidian Instrument Co., Ltd., Shanghai, China), following the protocols described in the Chinese national standard methods (State Environmental Protection Administration, Beijing, China, 1989) [45]. The concentrations of metal ions in the solution were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). Electron paramagnetic resonance (EPR) was used to detect the free radicals generated by the oxidation process. The specific operation was as follows: place a 2 mmol/L DMPO solution in the reaction system, add PDS for 30 s and then take a sample and cross the membrane; contain the filtrate in a 1.5 mL centrifuge tube; and then a small amount of filtrate is sucked up by capillary tube and put into the detector for detection. Test conditions: receiver gain 1 × 105; modulation amplitude 1 G, microwave power 10 Mw, modulation frequency 100 kHz. Additionally, solid precipitates obtained after the calcium precipitation experiments were characterized using X-ray diffraction (XRD, Rint2200, Rigaku Corporation, Tokyo, Japan) and scanning electron microscopy (SEM, JSM-6400, Jeol, Tokyo, Japan).

3. Results and Discussion

3.1. ED Regeneration of Wastewater

Since phosphite is the primary harmful contaminant in electroless nickel-plating wastewater, the purification efficiency of the ED process strongly depends on the mobility of phosphite ions. To optimize the process parameters, the present study investigated the effects of current density, current speed, sodium sulfate concentration in the concentrate chamber, and the volume ratio of concentrate with which to dilute solutions on the mobility of hypophosphite, phosphite, and nickel ions. In theory, nickel ions should remain in the dilute chamber due to the barrier effect of the anion–exchange membrane. However, since nickel ions in the plating wastewater mainly exist in complexed forms, a small fraction of nickel ions migrate across the membrane due to complexation with organic acids and other organics present in the wastewater [46].
It was observed that the current density was the predominant factor affecting ion mobility during the ED process. As illustrated in Figure 1a, increasing the current density significantly enhanced the mobility of hypophosphite and phosphite ions in the solution. When the current density was set at 20 mA/cm2, the mobility of hypophosphite and phosphite reached 67.3% and 62.5%, respectively. Further increases in the current density led to only marginal improvements in ion mobility. However, higher current densities substantially reduced the reaction time required for wastewater purification. Specifically, at a current density of 10 mA/cm2, approximately 120 min was required to complete the purification of 1 L of wastewater, whereas increasing the current density to 30 mA/cm2 shortened this time to 80 min. Nevertheless, excessive current densities may lead to overheating and the potential shutdown of the power supply. Moreover, elevated current densities increased the temperature of the plating solution, which subsequently lowered the migration efficiency of harmful ions, such as phosphite and sulfate.
As illustrated in Figure 1a, the mobility of Ni2⁺ ions increased from 2.37% to 9.4% with an increase in current density. Considering the practical circumstances and overall economic efficiency, the optimal current density was determined to be 20 mA/cm2. Figure 1b demonstrates that increasing the current speed gradually enhances the ion mobility, eventually reaching a stable plateau. At a current speed of 1.0 m3/h, ion mobility achieved its maximum stable value, and further increases in the current speed did not lead to significant improvements. Therefore, a current speed of 1.0 m3/h was recommended to maximize phosphite migration efficiency.
From Figure 1c,d, it can be observed that increasing the sodium sulfate concentration in the concentrate chamber slightly enhanced ion mobility; however, the improvement was not significant. Nevertheless, a higher sodium sulfate concentration effectively increased the initial conductivity in the concentrate chamber, thus shortening the reaction time. A comparative analysis of Figure 1c,d indicates that reducing the volume ratio of the concentrate to dilute water negatively affects ion mobility and significantly prolongs the reaction duration. Hence, considering these comprehensive factors, the recommended process parameters are a sodium sulfate concentration of 20 g/L in the concentrate chamber and a concentrate-to-dilute volume ratio of 1:1.

3.2. Reuse of Regenerated Wastewater

In the chemical nickel-plating process, the plating performance is significantly influenced by the phosphite concentration. Typically, plating performance deteriorates with the increasing phosphite concentration. Specifically, when the phosphite concentration reaches approximately 120 g/L, defects such as reduced coating adhesion and leakage become evident during thick plating, necessitating the termination of the plating process. In comparison, the thin plating processes can tolerate higher phosphite concentrations (up to approximately 200 g/L) before similar defects occur. In this study, considering that thick plating operations generally exhibit bonding issues and plating defects at lower phosphite concentrations (>120 g/L), thick plating was selected to evaluate the regeneration and reuse potential of the wastewater after ED treatment.
Figure 2 compares the performance of plated parts produced using the freshly formulated plating solution and the regenerated plating solution. As illustrated, parts plated using the regenerated solution exhibited slightly inferior performance relative to those plated with the freshly prepared bath, although the overall differences remained within an acceptable range. During the first three plating cycles, the regenerated plating bath consistently maintained coating thicknesses above 16.0 μm and deposition rates greater than 0.2 μm/min, fully meeting industrial production requirements. However, in the fourth cycle, plating quality decreased significantly, corresponding to an elevated phosphite concentration (>120 g/L) within the plating bath, indicating that the bath had accumulated excessive impurities and required disposal. Compared with the freshly prepared plating bath, which maintained stable performance for a longer duration, the regenerated plating bath effectively restored approximately 60% of the original activity.
In summary, the electrodialysis-based regeneration of aged nickel-plating solutions is technically viable, presenting substantial practical value and broad prospects for industrial application.

3.3. Recovery of Phosphorus from Concentrated Wastewater

3.3.1. Phosphorus Speciation Changes During Oxidation

Figure 3 illustrates the influence of the PDS concentration on phosphorus speciation during the oxidation reaction. As shown, increasing the PDS dosage resulted in significant changes in phosphorus speciation. Specifically, the hypophosphite concentration decreased continuously, accompanied by a corresponding increase in the orthophosphate concentration, consistent with expected oxidation trends. When observing the phosphite concentration, an initial increase followed by stabilization was noted. This phenomenon was attributed to the relatively slower oxidation kinetics of phosphite compared with hypophosphite. Initially, hypophosphite rapidly oxidized into phosphite, leading to an initial rise in the phosphite concentration, followed by a plateau phase. Subsequently, as hypophosphite oxidation neared completion, the oxidation of phosphite commenced at a comparatively slower rate at higher oxidant dosages and prolonged reaction times.
Although the complete oxidation of low-valent phosphorus (hypophosphite and phosphite) to orthophosphate was not fully achieved, the phosphorus recovery efficiency remained unaffected. This is attributed to the fact that both calcium phosphite and calcium phosphate are water-insoluble precipitates, effectively removable by calcium hydroxide precipitation. Therefore, the incomplete oxidation of phosphite does not impede phosphorus recovery, as both calcium phosphite and calcium phosphate can be effectively precipitated and removed from the solution.

3.3.2. Effects of Reaction Parameters

As shown in Figure 3, the oxidation efficiency of hypophosphite increased significantly with the increasing PDS dosage. Specifically, the oxidation efficiency rose from 48.04% to 99.8% when the PDS dosage was increased from 0.1 mol/L to 0.3 mol/L. A further increase in the PDS concentration resulted in negligible improvement, as the hypophosphite was already nearly completely oxidized at 0.3 mol/L. Hence, considering both cost and practical application, a PDS dosage of 0.3 mol/L is recommended for hypophosphite oxidation.
Figure 4b illustrates that the oxidation performance of various oxidants toward hypophosphite was similar under identical experimental conditions, with all tested oxidants eventually achieving near-complete oxidation. Figure 4c demonstrates the influence of temperature on the oxidation process. Considering industrial practicalities, the experiments were carried out within a common operational temperature range (10–40 °C). The results indicate that temperature primarily influences reaction kinetics; specifically, when the temperature exceeded 30 °C, hypophosphite was completely oxidized within 50 min.
Figure 4d depicts the effect of initial solution pH on hypophosphite oxidation. The results show that complete oxidation of hypophosphite occurred within approximately 1 h when the initial pH ranged between 3 and 7 at a PDS dosage of 0.3 mol/L. However, oxidation efficiency decreased under strongly acidic conditions (pH < 3), with an oxidation efficiency of only 84.15% at pH 1.0, indicating the significant inhibition of PDS activity. Furthermore, oxidation efficiency markedly decreased in alkaline conditions, with only 68.18% oxidation at pH 9.0. These findings confirm that the oxidation of hypophosphite by PDS is highly sensitive to solution pH, with optimal performance achieved in near-neutral conditions.
It was observed that solution pH significantly influenced the oxidation mechanism and efficiency of PDS. Electron paramagnetic resonance (EPR) experiments were performed to clarify the radical species involved during the oxidation process at different initial pH conditions (pH₀ = 3.0 and pH₀ = 11), and the results are presented in Figure 5. Signals corresponding to both DMPO–·OH and DMPO–SO4−· adducts were detected in both acidic (pH₀ = 3.0) and alkaline (pH₀ = 11) conditions, indicating the coexistence of SO4−· and ·OH radicals during PDS oxidation. However, under alkaline conditions (pH₀ = 11), a notably stronger signal for DMPO–·OH was detected, suggesting that SO4−· radicals were largely converted to ·OH radicals.
Considering the redox potentials of these two radicals (EOH = 2.80 eV, ESO4−· = 2.5–3.1 eV), the conversion of SO4−· to ·OH may decrease the overall oxidation capability of PDS under alkaline conditions [47,48]. Furthermore, the half-life of the ·OH radical (t1/2 < 1 µs) is significantly shorter than that of the SO4−· radical (t1/2 ≈ 4 s) [49,50], resulting in a substantial reduction in the radical lifetime, thus limiting any effective interaction with hypophosphite ions.

3.3.3. Effect of Calcium Dosage on Phosphorus Recovery

After oxidation treatment, phosphite and orthophosphate are present in the concentrated wastewater, and these phosphorus species can be precipitated by adding calcium salts to facilitate phosphorus recovery. To evaluate phosphorus recovery efficiency and optimize calcium salt selection, four different calcium salts were tested at a fixed Ca/P molar ratio of 2.0, as illustrated in Figure 6. The results indicate that CaO exhibited the lowest removal efficiency for TP and Ni2⁺ ions. In contrast, calcium hydroxide (Ca(OH)2) demonstrated superior removal efficiency for both TP and Ni2⁺. Although calcium chloride and calcium carbonate achieved moderate removal efficiencies, their addition introduced impure ions, complicating the subsequent wastewater treatment processes. In contrast, the use of Ca(OH)2 did not introduce extraneous ions, thereby simplifying downstream processing. Additionally, the Ca(OH)2 supernatant, which is strongly alkaline, can effectively neutralize the acidic electroplating wastewater commonly generated in industrial wastewater treatment facilities.
Thus, considering phosphorus and nickel removal efficiency, the absence of impure ions, and practical treatment convenience, Ca(OH)2 is recommended as the optimal calcium salt for phosphorus recovery.
Figure 7a illustrates the effect of the Ca/P molar ratio on TP removal from the concentrated wastewater. At low Ca/P ratios, the solution was weakly alkaline, which was not conducive to the effective precipitation of phosphite and phosphate species, resulting in a relatively lower phosphorus removal rate. As the Ca/P molar ratio increased, the phosphorus removal rate improved significantly, reaching an optimum of 98.75% at Ca/P = 2.0. Beyond this optimal ratio, further increases in calcium dosage led to minimal improvements in the phosphorus removal rate and even slightly reduced effectiveness. This reduction occurred because excessively high solution pH favored the formation of Ca(OH)2 precipitates over calcium phosphate and calcium phosphite, thus inhibiting phosphorus precipitation [51,52].
As shown in Figure 7b, the optimal Ni removal rate (93.42%) was also achieved at a Ca/P ratio of 2.0. Further increases in calcium dosage did not significantly enhance the nickel removal rate, and a residual nickel concentration (~40 mg/L) remained difficult to remove completely. Considering the combined removal rate of phosphorus and nickel, the Ca/P molar ratio of 2.0 was identified as optimal.
The impact of the reaction temperature on phosphorus removal is presented in Figure 7c. With increasing temperature, the phosphorus removal rate initially increased, peaking at 99.53% at approximately 55 °C, and subsequently declined slightly. Elevated temperature enhanced molecular diffusion, accelerating the precipitation reaction. However, excessively high temperatures caused the partial dissolution of loosely structured calcium phosphate crystals, releasing phosphate ions back into the solution and, thus, lowering the overall phosphorus removal rate. Even at room temperature (25 °C), the phosphorus removal rate reached 98.75%, indicating the sufficient availability of phosphorus species in the solution to facilitate rapid precipitation.
Figure 7d demonstrates the effect of temperature on the nickel removal rate. Increasing temperature gradually improved nickel removal, ultimately reaching a maximum removal rate of 94.77%, corresponding to a residual nickel concentration of approximately 20 mg/L.

3.3.4. Analysis of Precipitated Products

Figure 8 shows XRD patterns and scanning SEM images of the precipitates obtained after phosphorus recovery. The XRD analysis revealed that the precipitated products predominantly consisted of CaHPO3·H2O, along with minor amounts of hydroxyapatite (Ca5(PO4)3OH) and CaSO4·2H2O. Although Ni(OH)2 was theoretically expected to form, it was not detected by XRD, indicating that its concentration in the precipitate was very low. Considering the agricultural importance of phosphorus as a nutrient, the obtained precipitate has potential applications as a phosphorus fertilizer supplement. Additionally, minor impurities such as Ni(OH)2 and CaSO4·2H2O do not necessitate further separation, as they can function as soil conditioners, thus achieving the efficient utilization of recovered phosphorus resources.
The SEM analysis demonstrated that the precipitate consisted of particles of relatively uniform size. While most particles exhibited good dispersion, noticeable agglomeration into larger clusters was also observed.

4. Conclusions

In this study, an integrated approach combining ED and advanced oxidation processes was developed to achieve the efficient recovery of nickel and phosphorus resources from chemical nickel-plating wastewater. Taking into account the overall cost-effectiveness and operational simplicity, the recommended operational parameters for the ED process include a current density of 20 mA/cm2, a current speed of 1.0 m3/h, a sodium sulfate concentration of 20 g/L in the concentrate chamber, and a volume ratio of concentrate-to-dilute water of 1:1. Under these optimized conditions, the maximal migration of phosphite ions was achieved while minimizing the excessive loss of hypophosphite. The regenerated plating bath retained approximately 60% of its original plating activity, and the coating quality exhibited only minimal differences compared to freshly prepared plating solutions, demonstrating considerable potential for practical reuse.
PDS demonstrated outstanding oxidation performance for hypophosphite removal. At an optimal dosage of 0.3 mol/L, nearly the complete oxidation of hypophosphite ions was achieved. Reaction temperatures above 30 °C notably accelerated the oxidation reaction, fully oxidizing hypophosphite within 50 min. Furthermore, efficient hypophosphite oxidation was sustained within an initial pH range of 3–7, while oxidation efficiency was markedly inhibited under strongly acidic or alkaline conditions, indicating the clear pH sensitivity of the PDS oxidation process.
Additionally, phosphorus recovery was mostly effective at a calcium-to-phosphorus (Ca/P) molar ratio of 2.0, achieving removal efficiencies greater than 98% for TP and over 93% for nickel ions. The main components identified in the recovered precipitates included CaHPO₃·H₂O and hydroxyapatite, along with minor amounts of CaSO₄·2H₂O and Ni(OH)₂. These recovered precipitates could potentially be utilized directly as phosphorus fertilizers and soil conditioners, thus realizing resource recycling without necessitating further separation processes.
In summary, the innovative combined treatment process developed in this research provides a practical, straightforward, efficient, and economical solution for recovering valuable nickel and phosphorus resources from chemical nickel-plating wastewater. This proposed approach not only facilitates waste recycling but also aligns well with contemporary sustainable development principles and goals.

Author Contributions

Data curation, X.X.; Project administration, H.L. Software, X.X.; Supervision, W.W.; Validation, H.L.; Writing—original Draft, X.X. and K.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Science and Technology Innovation Tackle Plan Project of Anhui Province (Grant No.202423110050041).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

Author Wenming Wu was employed by the company Zhongxin Link Environmental Technology (Anhui) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Water 17 01071 g001
Figure 1. Effects of various parameters on ion mobility in the solution: ((a) effect of current density; (b) effect of current speed; (c) effect of sodium sulfate concentration in the concentrate chamber (concentrate-to-dilute ratio = 1:1); and (d) effect of sodium sulfate concentration in the concentrate chamber (concentrate-to-dilute ratio = 0.5:1)).
Figure 1. Effects of various parameters on ion mobility in the solution: ((a) effect of current density; (b) effect of current speed; (c) effect of sodium sulfate concentration in the concentrate chamber (concentrate-to-dilute ratio = 1:1); and (d) effect of sodium sulfate concentration in the concentrate chamber (concentrate-to-dilute ratio = 0.5:1)).
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Figure 2. Plating performance test. ((a) Plating thickness; (b) plating rate).
Figure 2. Plating performance test. ((a) Plating thickness; (b) plating rate).
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Figure 3. Effect of PDS concentration on changes in phosphorus morphology.
Figure 3. Effect of PDS concentration on changes in phosphorus morphology.
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Figure 4. Effects of various reaction parameters on the oxidation efficiency of hypophosphite: ((a) effect of PDS concentration; (b) comparison of different oxidants; (c) effect of reaction temperature; and (d) effect of initial solution pH).
Figure 4. Effects of various reaction parameters on the oxidation efficiency of hypophosphite: ((a) effect of PDS concentration; (b) comparison of different oxidants; (c) effect of reaction temperature; and (d) effect of initial solution pH).
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Figure 5. EPR spectra under different initial pH conditions.
Figure 5. EPR spectra under different initial pH conditions.
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Figure 6. Comparison of ion removal efficiencies using different calcium salts: ((a) TP removal rate; (b) Ni2⁺ removal rate).
Figure 6. Comparison of ion removal efficiencies using different calcium salts: ((a) TP removal rate; (b) Ni2⁺ removal rate).
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Figure 7. Effect of different parameters on ion removal efficiencies: ((a) effect of Ca/P molar ratio on TP removal rate; (b) effect of Ca/P molar ratio on Ni removal rate; (c) effect of reaction temperature on TP removal rate; and (d) effect of reaction temperature on Ni removal rate).
Figure 7. Effect of different parameters on ion removal efficiencies: ((a) effect of Ca/P molar ratio on TP removal rate; (b) effect of Ca/P molar ratio on Ni removal rate; (c) effect of reaction temperature on TP removal rate; and (d) effect of reaction temperature on Ni removal rate).
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Figure 8. Analysis of precipitation products. ((a) XRD image; (b) SEM image).
Figure 8. Analysis of precipitation products. ((a) XRD image; (b) SEM image).
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Table 1. Comparative summary of existing technologies.
Table 1. Comparative summary of existing technologies.
Processing TechnologyKey AchievementsDrawbacks
chemical precipitationRemarkable treatment effect; mature process; simple operation; wide range of applicationsProduces large amounts of sludge; difficult to completely remove complexed nickel; stringent pH control requirements
advanced oxidation processesStrong broad-spectrum degradation ability; highly efficient complex-breaking and destabilization; suitable for deep treatmentHigher operating costs; harsh reaction conditions; complex equipment and high maintenance requirements
electrochemical methodsHigh degree of automation control; green environmental protection; no secondary pollution; suitable for the recovery of valuable metalsHigher equipment and power costs; electrodes are susceptible to passivation or corrosion; pH- and conductivity-sensitive
ion exchangeStable and clear effluent quality; resource recycling; easy to operate; can be automatedThe resin easily becomes poisoned or clogged; the scope of application is limited; there are difficulties in the treatment of regeneration fluid.
adsorptionHighly efficient removal of nickel ions (free and complexed); high green potential; suitable for low concentrations of wastewater treatmentLimited adsorption capacity, which needs to be replaced or regenerated frequently; regeneration operation is complicated and may lead to secondary pollution; poor treatment of high concentrations of wastewater
membrane separationCompact process with small footprint; no additives; no chemical reaction; adaptable and applicable to a variety of pollutantsSerious membrane contamination problems; high investment and operating costs; high requirements for influent water quality
biological treatmentsGreen environmental protection with no secondary pollution; produces a wide source of materials, making it renewable; low concentration of nickel has a good removal abilityDifficulty to achieve rapid industrialization and diffusion; stringent requirements for environmental conditions; regeneration and longevity of biomaterials
Table 2. Composition of chemical nickel-plating wastewater.
Table 2. Composition of chemical nickel-plating wastewater.
ItemspHNi2+
(g/L)		H2PO2−
(g/L)		HPO32−
(g/L)		PO43−
(g/L)		TP
(g/L)
Value4.56 ± 0.25.82 ± 0.127.91 ± 2124.96 ± 50.1687 ± 0.2190.38 ± 5
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Xiong, X.; Cui, K.; Li, H.; Wu, W. Green Regeneration and Resource Recovery of Nickel-Plating Waste Solution: A Synergistic Study of Electrodialysis and Advanced Oxidation. Water 2025, 17, 1071. https://doi.org/10.3390/w17071071

AMA Style

Xiong X, Cui K, Li H, Wu W. Green Regeneration and Resource Recovery of Nickel-Plating Waste Solution: A Synergistic Study of Electrodialysis and Advanced Oxidation. Water. 2025; 17(7):1071. https://doi.org/10.3390/w17071071

Chicago/Turabian Style

Xiong, Xiaolong, Kangping Cui, Haiyang Li, and Wenming Wu. 2025. "Green Regeneration and Resource Recovery of Nickel-Plating Waste Solution: A Synergistic Study of Electrodialysis and Advanced Oxidation" Water 17, no. 7: 1071. https://doi.org/10.3390/w17071071

APA Style

Xiong, X., Cui, K., Li, H., & Wu, W. (2025). Green Regeneration and Resource Recovery of Nickel-Plating Waste Solution: A Synergistic Study of Electrodialysis and Advanced Oxidation. Water, 17(7), 1071. https://doi.org/10.3390/w17071071

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